The subject of the invention is a device for measuring voltages by sampling which can be used in particular by independent instruments using broad frequency ranges or by automatic testing equipment.
For high-frequency voltage meters the selected mode of operation generally determines and thereby restricts the range of applications of the measuring instruments. The number of systems that can be chosen is further restricted when automatic testing equipment are being used since for these instruments, which have an upper frequency limit of ghz magnitude, it is necessary to guarantee rapid operation, great precision, and proper temporal as well as thermal stability.
Two types of models are known for programmable high-frequency meters which can also be used in automatic testing equipment: One works with a detector and one in accordance with the principle of incoherent (stochastic) sampling. Theoretically about the same frequency range can be reached with these two types of instruments.
For instruments with a detector, the high-frequency signal to be measured arrives at the input to the detector circuit, constructed of diodes. The DC signal appearing at the output of the detector circuit is proportional for small input levels to the true effective value of the input signal because of the characteristics of the diodes; however, for large input levels it is proportional to the peak value of the input signal. The transmission characteristic of the detector circuit is non-linear, and transformation effectiveness rapidly decreases in the direction of small levels. The output signal of the detector circuit arrives, after appropriate amplification, at the input of a non-linear amplifier. The non-linear amplifier compensates for the non-linear detector circuit. The output signal of the non-linear amplifier can be connected to an instrument or, by an analog-digital converter, to a digital indicator or, for further processing, to a computer.
Using this principle for measuring has the following disadvantages:
the non-linear transmission characteristics of the detector must be linearized with a non-linear break point approximation circuit which requires a difficult adjustment which depends on the individual diodes;
in case of large and small input levels, the output signal of the detector circuit is always proportional to different parameters of the input signal (effective value, peak value); measurements of complex signals (e.g., noise) with variable level cannot, therefore be clearly evaluated;
when input levels are low, the transformation effectiveness decreases very rapidly because of the characteristics of the diodes; the instrument, consequently, becomes insensitive below an input level of about 200 mv and, as a result, the instrument cannot be readjusted; only an adjustment range can be indicated.
The operation of an instrument functioning in accordance with the principle of incoherent sampling occurs as follows: The high-frequency signal to be measured arrives at the input point of a sampler. For a very short time the sampler switches the signal to be measured, corresponding to the frequency of an voltage-controlled oscillator, to the input of an amplifier with constant amplification. The sampling may last some 100 psec; thus, the input capacity of the amplifier is loaded with proportional voltage over the entire frequency spectrum of the instrument with the instantaneous value of the signal to be measured while it is being discharged between two samples. The amplitude of the impulse sequence appearing at the output of the amplifier is, therefore, proportional to the instantaneous value of the signal to be measured at the moment of sampling. When the frequency of the sampling stochastically changed in relation to the signal to be measured which is to say that the sampling is incoherent and that a large number of sampled values of the signal to be measured can be obtained the statistical parameters of the sampled impulse sequence (peak, medium, and effective value) are the same as the corresponding values of the signal to be measured (principle of stochastic sampling). Stochastic sampling occurs in such a way that the frequency of the voltage-controlled oscillator controlling the sampling is frequency modulated by another oscillator.
The device in accordance with the principle of incoherent sampling has the following advantages over the detector type:
the transmission characteristic of the sampler is linear because the sampling switch is controlled by a constant-amplitude signal; the transformation effectiveness, consequently, is not a function of the amplitude of the signal to be measured;
the output signal of the sampler has the statistical parameters of the signal to be measured, regardless of input level; consequently when measuring complex signals (e.g., noise) the measurement can be precisely evaluated;
since the opening of a switch of the sampler is carried out by a signal that is independent of the signal to be measured, the transformation effectiveness of the sampler is independent of the amplitude of the signal to be measured; thus, the device can be precisely calibrated.
The disadvantage of a device which works in accordance with the principle incoherent sampling is that the transformation effectiveness of the sampler is significantly dependent on outside temperature, which has the apparent effect of a change in amplification. This value is approximately three to four times as high as the characteristic value for devices operating with diode detectors. This problem is solved by a known method in such a way that the device has a built-in calibration instrument at the output to which the measured signal input can be linked, and the device can thus be calibrated in accordance with the prevailing outside temperature. This calibration device is really a reference standard which makes it rather expensive. An additional disadvantage of this solution is that, when it is used in an automatic testing equipment, it obviously increases the duration of said sampling device's measurements significantly because of temporary disruption of the testing process.